Cytological diagnosis of oncocytic variant of medullary thyroid carcinoma with histological and immunohistochemical correlation Running title: Cytolomorphology of oncocytic variant of medullary thyroid carcinoma

Abstract Medullary thyroid carcinoma (MTC) is a sporadic or familial tumor of the parafollicular or C-cells that secrete calcitonin. Oncocytic tumors of the endocrine system are a group of rare entities described in several organs, including the thyroid gland and seen in a variety of thyroid conditions that are associated with a broad differential diagnosis like benign (Hürthle cell adenoma, granular cell tumor) and malignant (Hürthle cell carcinoma) neoplasms, variants of papillary thyroid carcinoma (PTC) (tall cell variant, oncocytic variant, and Warthin-like variant) and the oncocytic variant of medullary carcinoma. Oncocytic variant of medullary carcinoma is exceptional not only because of being extremely rare but also being the only oncocytic tumor that is not derived from follicular cells of the thyroid gland. We report here an 18-year-old female patient who presented with complaints of passage of loose stool for 4 months and was on examination detected to have a right thyroid nodule. FNAC of the nodule was done and an oncocytic tumour was diagnosed. The differentials considered were Hürthle cell lesion and medullary carcinoma thyroid (oncocytic cell variant). This was followed by total thyroidectomy along with modified radical neck dissection. Histopathological examination and immunohistochemistry were performed in addition to serum calcitonin levels and a diagnosis of oncocytic variant of medullary carcinoma thyroid was confirmed. We hereby conclude that MTC should be considered as one of the differentials for accurate diagnosis in unusual thyroid carcinomas with predominance of oncocytic cell changes.

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